Key out background and keep "halo" around foreground
You don't need to key the text. Just multiply it over the background image.

For legibility you can add a fake drop shadow, by multiplying a version of the same text displaced and blured:

Or if you need white, then use the dilate/erode node in feather mode, invert the result,  and use color dodge to add a soft white border around the letters.

Or use the black writing as a mask to have color text.